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dirfile-encoding — dirfile database encoding schemes
The Dirfile Standards indicate that RAW fields defined in the database are accompanied by binary files containing the field data in the specified simple data type. In certain situations, it may be advantageous to convert the binary files in the database into a more convenient form. This is accomplished by encoding the binary file into the alternate form. A common use-case for encoding a binary file is to compress it to save disk space. Only data is modified by an encoding scheme. Database metadata is never encoded. Support for encoding schemes is optional. An implementation need not support any particular encoding scheme, or may only support certain operations with it, but should expect to encounter unknown encoding schemes and fail gracefully in such situations. Additionally, how a particular encoding is implemented is not specified by the Dirfile Standards, but, for purposes of interoperability, all dirfile implementations are encouraged to support the encoding implementation used by the GetData dirfile reference implementation, elaborated below. An encoding scheme is local to the particular format specification fragment in which it is indicated. This allows a single dirfile to have binary files which are stored using multiple encodings, by having them defined in multiple fragments. The rest of this manual page discusses specifics of the encoding framework implemented in the GetData library, and does not constitute part of the Dirfile Standards.
THE GETDATA ENCODING FRAMEWORK
The GetData library provides an encoding framework which abstracts binary file I/O, allowing for generic support for a wide variety of encoding schemes. Functions which may make use of the encoding framework are: gd_add(3), gd_add_raw(3), gd_add_spec(3), gd_alter_encoding(3), gd_alter_endianness(3), gd_alter_frameoffset(3), gd_alter_entry(3), gd_alter_raw(3), gd_alter_spec(3), gd_flush(3), gd_getdata(3), gd_malter_spec(3), gd_move(3), gd_nframes(3), gd_putdata(3), gd_raw_close(3), gd_rename(3), and gd_sync(3). Most of the encodings supported by GetData are implemented through external libraries which handle the actual file I/O and data translation. All such libraries are optional; a build of the library which omits an external library will lack support for the associated encoding scheme. In this case, GetData will still properly identify the encoding scheme, but attempts to use GetData for file I/O via the encoding will fail with the GD_E_UNSUPPORTED error code. GetData discovers the encoding scheme of a particular RAW field by noting the filename extension of files associated with the field. Binary files which form an unencoded dirfile have no file extension. The file extension used by the other encodings are noted below. Encoding discovery proceeds by searching for files with the known list of file extensions (in an unspecified order) and stopping when the first successful match is made. Because of this, when the a field has multiple data files with different, supported file extensions which could legitimately be associated with it, the encoding scheme discovered by GetData is not well defined. In addition to raw (unencoded) data, GetData supports nine other encoding schemes: text encoding, bzip2 encoding, flac encoding, gzip encoding, lzma encoding, sie (sample-index encoding), slim encoding, zzip encoding, and zzslim encoding, all discussed below. The text encoding and the sample-index encoding are implemented by GetData natively and need no external library. As a result, they are always present in the library. Out-of-place writes Some of the encodings listed below only support writing via out-of-place writes; that is, raw files are written in a temporary location and only moved into place when closed. As a result, writing to these encodings requires making a copy of the whole binary data file. A further side effect of this is that a third-party trying to concurrently read a Dirfile which is being written to using one of these encodings usually doesn't work. Within GetData, reading from a field so encoded after writing to it will cause writing to the temporary file to be finished and then the file moved into place before the read occurs, which may take some time to do. Encodings which perform out-of-place writes are: bzip2, flac, gzip, and lzma. BZip2 Encoding The BZip2 Encoding reads compressed raw binary files using the Burrows-Wheeler block sorting text compression algorithm and Huffman coding, as implemented in the bzip2 format. GetData's BZip2 Encoding scheme is implemented through the bzip2 compression library written by Julian Seward. All operations are supported by the BZip2 Encoding, but writing occurs out-of-place. See the Out-of-place writes section above for details. GetData caches an uncompressed megabyte of data at a time to speed access times. A call to gd_nframes(3) requires decompression of the entire binary file to determine its uncompressed size, and may take some time to complete. The file extension of the BZip2 Encoding is .bz2. FLAC Encoding The FLAC Encoding compresses raw binary files using the Free Lossless Audio Codec. GetData's FLAC Encoding scheme is implemented through the libFLAC reference implementation developed by Josh Coalson and the Xiph.Org Foundation. All operations are supported by the FLAC Encoding, but writing occurs out-of-place. See the Out-of-place writes section above for details. The FLAC format only permits samples up to 32-bits, but the libFLAC reference codec can only handle samples up to 24-bits. GetData gets around this by slicing data that is wider than 16-bits into multiple channels (2, 4, or 8, depending on width). For big-ended data, the most-significant 16-bits are in channel 0, the second 16-bits in channel 1, &c. For little-ended data, this is reversed, with the least significant word in channel 0. The sample rate specified in the FLAC header is ignored and may be any valid value. FLAC files written by GetData use a sample rate of 1 Hz. The file extension of the FLAC Encoding is .flac. The Ogg container format is not supported. GZip Encoding The GZip Encoding compresses raw binary files using Lempel-Ziv coding (LZ77) as implemented in the gzip format. GetData's GZip Encoding scheme is implemented through the zlib compression library written by Jean-loup Gailly and Mark Adler. All operations are supported by the GZip Encoding, but writing occurs out-of-place. See the Out-of-place writes section above for details. To speed the operation of gd_nframes(3), the GZip Encoding takes the uncompressed size of the file the gzip footer, which contains the file's uncompressed size in bytes, modulo 2**32. As a result, using a field with an (uncompressed) binary file size larger than 4 GiB as the reference field will result in the wrong number of frames being reported. The file extension of the GZip Encoding is .gz. LZMA Encoding The LZMA Encoding reads compressed raw binary files using the Lempel-Ziv Markov Chain Algorithm (LZMA) as implemented in the xz container format. GetData's LZMA Encoding scheme is implemented through the lzma library, part of the XZ Utils suite written by Lasse Collin, Ville Koskinen, and Igor Pavlov. All operations are supported by the LZMA Encoding, but writing occurs out-of-place. See the Out-of-place writes section above for details. Writing is supported only for the .xz container format, and not for the obsolete .lzma format, which can still be read. GetData caches an uncompressed megabyte of data at a time to speed access times. A call to gd_nframes(3) requires decompression of the entire binary file to determine its uncompressed size, and may take some time to complete. The file extension of the LZMA Encoding is .xz, or .lzma. Sample-Index Encoding The Sample-Index Encoding (SIE) compresses raw binary data by replacing runs of repeated data, similar to run-length encoding. SIE files contain binary records consisting of a 64-bit sample number followed by a datum (the size and format of which is determined by the RAW field's data type in the format metadata). The sample number indicates the last sample of the field which has the specified value. The first sample with the value is the sample immediately following the data in the previous record, or sample number zero, for the first record. Sample numbers are relative to any /FRAMEOFFSET specified in the Dirfile metadata. All operations are supported by the Sample-Index Encoding. The file extension of the Sample-Index Encoding is .sie. Slim Encoding The Slim Encoding reads compressed raw binary files using the slimlib compression library written by Joseph Fowler. The slimlib library was developed at Princeton University to compress dirfile-like data. GetData's Slim Encoding framework currently lacks write capabilities; as a result, the Slim Encoding does not support function which modify binary files. The file extension of the Slim Encoding is .slm. Using the Slim Encoding with GetData may result in unexpected, but manageable, memory usage. See the gd_getdata(3) manual page for details. Text Encoding The Text Encoding replaces the binary data files with 7-bit ASCII files containing a decimal text encoding of the data, one sample per line. All operations are supported by the Text Encoding. The file extension of the Text Encoding is .txt. ZZip Encoding The ZZip Encoding reads compressed raw binary files using the DEFLATE algorithm as implemented in the PKWARE ZIP archive container format. GetData's ZZip Encoding scheme is implemented through the zzip library written by Tomi Ollila and Guido Draheim. The ZZip Encoding framework currently lacks write capabilities; as a result the ZZip Encoding does not support functions which modify binary data. Unlike most encoding schemes, the ZZip encoding merges all binary data files defined in a given fragment into a single ZIP archive. The name of this archive is raw.zip by default, but a different name may be specified using the second parameter to the /ENCODING directive. For example, /ENCODING zzip archive indicates that the ZIP archive is called archive.zip. The file extension of the ZZip Encoding is .zip. ZZSlim Encoding The ZZSlim Encoding is a convolution of the Slim Encoding and the ZZip Encoding. To create ZZSlim Encoded files, first the raw data are compressed using the slim library, and then these slim-compressed files are archived (and compressed again) into a ZIP archive. As with the ZZip Encoding, the ZIP archive is raw.zip by default, but a different name may be specified with the /ENCODING directive. Notably, since the archives have the same name as ZZip Encoded data, automatic encoding detection on ZZSlim Encoded data always fails: they are incorrectly identified as simply ZZip Encoded. As a result, an /ENCODING directive in the format file or else a GD_ZZSLIM_ENCODED flag passed to gd_open(3) is required to read ZZSlim encoded data. The file extension of the ZZSlim Encoding is .zip. Using the ZZSlim Encoding with GetData may result in unexpected, but manageable, memory usage. See the gd_getdata(3) manual page for details.
This manual page was written by D. V. Wiebe <email@example.com>.